In well intervention operations using a string spooled externally onto drum while using an intervention string laying guide imposing a fleet angle is used. The laying guide shifts the intervention string laterally and imposes a fleet angle on the string. A fleet angle variation with otherwise constant drum rotation speed and string speed usually incurs a tension variation which is undesired. Several of the published patents comprise a drum with a pivotable rotational the drum.
U.S. Pat. No. 3,524,606 Cable reel mounting, describes a drum for lowering and hauling a cable through a set of horizontal guide rollers on top of a vertical pipe. The drum axis is arranged pivotable so as for allowing the cable to run tangentially between the drum and the rollers for all lateral positions of the cable on the drum, so as for reducing fleet angle variation of the cable relative to the drum.
U.S. Pat. No. 3,690,409 describes another pivotable drum arranged for keeping the fleet angle close to the perpendicular line of the drum by shifting the ends of the drum axially while the cable is wound or unwound. An advantage of U.S. Pat. No. 3,690,409 is an increased allowable axial length of the drum and thus an increased cable length capacity.
GB2296001 describes a winch apparatus for deploying or taking in line over a pulley arranged at a distance from the drum. The drum axis is pivotable so as for maintaining the fleet angle of the line near the perpendicular.
WO2006/027553 Richards describes a drum wherein the incoming line runs via a diamond screw controlled line guide which lays the cable with a fleet angle nearly perpendicularly on the drum. The diamond screw controlled line guide allows for the line to be directed parallel with the drum axis.
DE19942608 Becker describes a winch with an axially translating wire drum with a single ply. The axial translation for the wire drum is for guiding the wire in through a fixed entry position of the winch, while the drum is alternating along its axis.
WO2010/117162 also describes such an axially translating drum in a frame with a fixed entry point.
U.S. Pat. No. 7,753,344 to Moretz also describes an axially translating drum in a winch housing with a centrally arranged fixed entry position on the housing.
U.S. Pat. No. 2,810,439 McCullough describes a wellhead winch with an axially translating drum in a winch housing, wherein the winch housing is arranged for being connected under pressure to the wellhead.
EP0571207 describes a winch assembly with a translating drum and a fairlead for guiding the wire onto the drum under a desired fleet angle.
Another problem in the prior art is related to relative speed variations between the drum and the injector head. If we try and stop an intervention string during a hauling operation running the string out of the well, we run the risk of damaging the cable or the injector head very quickly. This is due to the inertia of the different components involved, because the injector head motors move less mass and are much faster to respond than the spooling unit motor which rotates a drum of considerable rotational inertia. Conversely, while feeding the intervention string into the well and suddenly stop it, the rotational inertia of the large drum with its coiled-up string means that it will continue to try and give out cable despite the injector head has already stopped the cable. The usual way of compensating for such speed differences often used in coiled tubing rig ups is to let the coiled tubing travelling through the air, so when it is stopped or started quickly the length of the free air arch changes to compensate. However a free travel of the intervention string hanging in an arch between the injector head's gooseneck and the spooling unit may not be desirable both from safety considerations both to operators or mechanical damage, particularly when the distance is large and swinging movements of the string may be considerable.
If an internal laying drum is used, and the injection head is running the cable into the well and suddenly stops, the inertia of the large drum means that it will continue to try and give out the rigid cable even though the injector head has already stopped. This may damage the cable through longitudinal compression with subsequent buckling or dislocation.